1. Technical Field
This invention relates to an optical time domain reflectomer (OTDR) and a method of fault or break detection by optical line domain reflectometry.
Optical communication systems are increasingly relying on the use of optical amplifiers to boost the optical signals so that communication at high bit rates over long ranges can be achieved. The cable and amplifiers for such systems are often located in inaccessible places, so a method of fault finding remotely from the ends of the system is required.
2. Description of the Prior Art
One method for measuring faults in fiber spans which has been used extensively is the optical time domain reflectometer. This injects an optical pulse into the fiber and then measures the relative magnitude and delay of the back scatter from the pulse as it propagates through the fiber. Generally optical amplifiers used in commercial applications include an optical isolator which prevents the backscattered light from passing back through the amplifier. The isolator therefore precludes the use of this type of OTDR on fiber spans employing optical amplifiers.
Several methods have been proposed for fault location on a bidirectional optically amplified line e.g. R. Jensen, C. Davidson, D. Wilson and J. Lyons. "Novel Technique for Monitoring Long Haul Undersea Optical Amplifier Systems", Conference on Optical Fiber Communications 1994, Paper No. ThR3. The most convenient is the inclusion of cross couplers between the send and return amplifiers within a repeater as illustrated in FIG. 1. Note: Amplifiers A & C are an amplifier pair in one repeater, amplifiers B & C are an amplifier pair in the next repeater in the line.
Conventional OTDR's using an amplitude modulation pulse stream are not suitable for use in a system employing optical amplifiers since the amplifiers require a constant mean power with respect to the erbium fiber time constant. To overcome this it has been proposed by Y. Horiuchi, S. Ryu, K. Mochizuki and H. Wakabayashi, in "Novel Coherent Hetrodyne Optical Time Domain Reflectometry for Fault Localization of Optical Amplifier Submarine Cable Systems". IEEE Photonics Technology Letters, Vol 2, No. 4 (April 1990), that the pulses used are in the form of frequency shift keying (FSK) of an optical carrier (as in FSK RADAR).
Such systems suffer from degradation due to polarization effects and the present invention seeks to reduce the effect of polarization.